This invention relates to a digital mixing apparatus. It arose in the design of an intermediate frequency digitizer for a radar receiver but would be equally applicable in other fields such as telecommunications.
A conventional arrangement for converting received radar signals into digital form is shown in FIG. 1. A signal received by an antenna 1, typically having a frequency of several GHz, is mixed at 2 with a local oscillator signal from 3. The output of the mixer 2 is passed through a bandpass filter 4. The output of the filter is at an intermediate frequency of, say, 60 MHz. This signal is passed to in phase (P) and quadrature (Q) channels generally indicated at 5 and 6 where it is mixed at 7 and 8 with a waveform from a second local oscillator 9 which also operates at 60 MHz and has its output phase shifted by 90 degrees at 10 before passing to the mixer 8. The outputs of the mixers 7 and 8 are passed through low-pass filters 11 and 12 and are amplified at 13 and 14. They are then converted into digital form by analogue-to-digital converters 15 and 16 clocked by a clock 17. The two outputs are the real and imaginary components of complex numbers representing samples of the signal at the frequency of the clock 17.
In a conventional system such as shown in FIG. 1 it is important that the mixers 7 and 8, filters 11 and 12, amplifiers 13 and 14 and analogue to digital converters 15 and 16 have exactly the same characteristics in each channel 5 and 6 and that the phase shift introduced at 10 be exactly 90 degrees. Otherwise the signal will be distorted. It can be troublesome and expensive to ensure that these requirements are met and remain so despite possible effects of ageing of the components.